memcg: get rid of mem_cgroup_root_css for !CONFIG_MEMCG

The only user is cgwb_bdi_init and that one depends on
CONFIG_CGROUP_WRITEBACK which in turn depends on CONFIG_MEMCG so it
doesn't make much sense to definte an empty stub for !CONFIG_MEMCG.
Moreover ERR_PTR(-EINVAL) is ugly and would lead to runtime crashes if
used in unguarded code paths.  Better fail during compilation.

Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: Vladimir Davydov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index ab2f6880e27b2d95264bd51712adcf52af6ebfe9..91bbe4ba6233686668e5ebba37436c36dbc62536 100644 (file)
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -498,8 +498,6 @@ void mem_cgroup_split_huge_fixup(struct page *head);
 #else /* CONFIG_MEMCG */
 struct mem_cgroup;
 
-#define mem_cgroup_root_css ((struct cgroup_subsys_state *)ERR_PTR(-EINVAL))
-
 static inline void mem_cgroup_events(struct mem_cgroup *memcg,
                                     enum mem_cgroup_events_index idx,
                                     unsigned int nr)